This application claims the benefit of Japanese Patent Application No. 2001-311795, filed Oct. 9, 2001 in the Japanese Intellectual Property Office, the disclosure of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a power source circuit, and more particularly, to a power source circuit used in an electrophotographic printer.
2. Description of the Related Art
FIG. 1 is a circuit diagram illustrating a conventional power source circuit. In general, the conventional power source circuit has a self-oscillation circuit at a primary side of a transformer 10. According to the operation of a fly wheel diode 30, which is connected to a switching transistor 20 in the self-oscillation circuit in parallel or embedded in the switching transistor 20, a primary fly back pulse, which is input to the primary side of the transformer 10, becomes a waveform similar to half of a sine wave. Since the transformer 10 boosts the waveform similar to the half of a sine wave, a secondary side of the transformer 10 also outputs a waveform similar to the waveform occurring on the primary side of the transformer 10. The waveform output from the secondary side of the transformer 10 is input to a voltage doubler circuit.
In the conventional power source circuit, the switching transistor may operate in a linear region, i.e., an active region, so that a problem of power loss, for example, the generation of heat, occurs in the switching transistor.
In addition, since the waveform similar to the half of a sine wave is output from the secondary side of the transformer and the output waveform is input to the voltage doubler circuit connected to the secondary side of the transformer, the voltage utility efficiency of the voltage doubler circuit deteriorates. More specifically, the waveform similar to the half of a sine wave obtains only half an amount of the transformer output voltage.
Further, where the coil ratio of the transformer increases in order to output a high voltage from the secondary side of the transformer, the voltage loss due to the distribution capacity of coils is increased and the boosting efficiency where the transformer boosts a voltage is lowered.
To solve the above-described and other problems, an object of the present invention is to provide a power source circuit for reducing power loss in a switching transistor, improving the voltage utility efficiency of a circuit connected to the secondary side of a transformer, and preventing a deterioration of boosting efficiency where the transformer boosts a voltage.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and, in part, will be obvious from the description, or may be learned by practice of the invention.
To achieve the above and other objects of the invention, according to one aspect of the present invention, there is provided a power source circuit comprising a transformer having a primary coil, a secondary coil, and a feedback coil, a resonance capacitor connected in parallel with the primary coil of the transformer, a switching unit having an input terminal connected to one end of the feedback coil of the transformer and an output terminal connected to one end of the primary coil of the transformer and controlling a current, which flows through the output terminal, according to a voltage input to the input terminal, and a current detection unit which detects the current, which flows through the switching unit, in order to turn off the switching unit when the detected current is higher than a predetermined current. Here, the end of the feedback coil connected to the input terminal of the switching unit and the end of the primary coil connected to the output terminal of the switching unit have different polarities.
Since the switching unit, for example, a switching transistor, operates in a switching region, i.e., a saturation region, a collector loss in the switching transistor is reduced. In other words, a collector voltage increases while a sufficient amount of base current flows so that the switching transistor operates in a switching region.
The current detection unit may comprise a resistor connected in series with the switching unit and a voltage clamp unit connected to a control terminal of the switching unit.
The power source circuit may further comprise a current direction restriction unit for restricting the direction of the current which flows between the end of the primary coil of the transformer and the output terminal of the switching unit.
A waveform input to the primary side of the transformer may be similar to a sine wave so that the waveform of a voltage output from the secondary side of the transformer is similar to the sine wave. Therefore, since the waveform similar to the sine wave is input to a circuit, which is connected to the secondary side of the transformer, the voltage utility efficiency of the circuit connected to the secondary side of the transformer improves.
The power source circuit may further comprise an output voltage stabilization circuit for controlling a voltage, which is input to the input terminal of the switching unit, according to the output voltage from the secondary coil of the transformer.
Accordingly, the voltage output from the power source circuit is stabilized.